This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Transporter proteins perform a critical role, actively relaying molecules across the membrane barrier and against their concentration gradient. The goal of this project is to understand how these proteins act on a molecular level. This requires detailed structural knowledge coupled with dynamic information, including the timescale, amplitude and direction of structural changes, in order to build a 'movie'of the protein in action. NMR is uniquely suited to address these questions since kinetic data are measured simultaneously with the chemical shift, a direct manifestation of atomic resolution structure. The small multidrug resistance efflux protein, EmrE, provides an ideal model system to examine how molecular motion is coupled to a directed biological response. EmrE couples proton import to drive polyaromatic cation export in E. coli, thus conferring resistance to drugs of this type. Although the details are not known, protein conformational change must occur for proper transport, allowing alternating access to either side of the membrane in response to substrate binding. This project investigates the proposed transport model and the mechanism of the critical coupling between binding and transport in EmrE. This knowledge will aid efforts to combat the contribution of multidrug resistance transporters to bacterial antibiotic resistance.